1. Field of the Invention
This invention relates to a flexible and automated optical inspection system (AOI) of focussed multiple angle illumination and reflection.
More particularly, this invention relates to an illumination system used in surface analyzers for printed circuit boards (PCBs). The invention employs automated multiple angle illumination and reflection for the analysis of surface contrasts on PCBs and to detect variations from a predetermined norm. The system and method of the invention is also potentially useful in the photocopier, microscopy and facsimile arts and indeed in other arts wherein optically characterizing contrasts of images on an object is important.
2. Background Art
The optical section of a typical AOI system comprises three main sections: imaging lens; line scan camera/camera controller; illumination system. The line scan camera includes a linear array of photodetectors known as pixels. The pixels store energy corresponding to the light intensity scattered by the object and collected by the imaging lens.
The object scattering pattern, of course, varies with the material and the processing conditions. Likewise the defect set varies with the manufacturing process and the circuit board performance specification. To achieve consistent, reliable results, the scattered light variation must be compensated by adjusting the illumination incidence angle, the illumination intensity, or a combination of incidence angle and intensity.
During the inspection process, the circuit board travels at constant velocity perpendicular to the line scan camera. An image within the width of the camera scan and the length of the circuit board, is collected by the line scan camera. The optical system is translated the width of the camera scan and another sweep of the board is taken. This process is repeated until the image of the entire board is acquired.
Optical surface analysis systems are described in the art. U.S. Pat. No. 4,498,778 issued Feb. 12, 1985 to White describes a system for locating points, as opposed to defects, on a workpiece. The spatial coordinates of the workpiece are determined in a system that includes illumination by a laser at a single, constant and known angle of incidence, the reflection being viewed also at a single angle.
U.S. Pat. No. 4,570,180 issued Feb. 11, 1986 to Baier et al. describes a meander scanning method comprising two steps, the first in which a digitized, stored image is scanned for edges or lines (transitions) and marked. In the second step the unmarked regions are scanned for permissible gray ares. Illumination is not discussed.
U.S. Pat. No. 4,650,333 issued Mar. 17, 1987 to Crabb et al. describes the detection of holes and nodules using single laser incidence and reflection, at a constant angle.
U.S. Pat. No. 4,754,329 issued Jun. 28, 1988 to Lindsay et al. describes the focusing and calibrating of a display screen rather than the illumination of the workpiece.
U.S. Pat. No. 4,758,782 issued Jul. 19, 1988 to Kobayashi describes improvements to the analysis of data obtained by combining feature extraction and mutual comparison methods, concentrating on the computer function and measuring electronics rather than the illumination.
U.S. Pat. No. 4,794,647 issued Dec. 27, 1988 to Forgues et al. describes the simultaneous dimensional verification and pattern recognition used in a probability weighing method of detecting flaws. The illumination is described as emanating from one light source above and a second light source below the workpiece, and one or more cameras above.
U.S. Pat. No. 4,799,175 issued Jan. 17, 1989 describes the table carriage and data manipulation systems after image acquisition rather than the illumination step, the problem addressed being problems in consistent workpiece alignment.
U.S. Pat. No. 4,811,410 issued Mar. 7, 1989 to Amir et al. describes an inspection of electronic devices of the kind mounted on PCBs. Non-uniformity of illumination is compensated for by weighting the threshold value of each pixel rather than by varying the illumination parameters.
U.S. Pat. No. 4,878,114 issued Oct. 31, 1981 to Huynh et al. describes a method of measuring the roughness of the surface of paper or of the finish of a machined part using light incident at an optimal grazing angle and its reflection, preferably through microscope optics, to a video camera.
U.S. Pat. No. 4,893,932 issued Jan. 16, 1990 to Knollenberg describes a method of surface analysis using the split beam of light reflected from two laser beams.
However, nowhere in the art is the AOI system and method of the present invention described, nor is, in particular, the illumination part of the system and method of the present invention.
Various systems are known in the art for determining the contour of an object by viewing the object at an angle from an illuminating beam, and comparing the object with a stored standard. Other systems provide an unacceptably high number of false defect calls due to trivial surface features such as stains and minor scratches.
The present invention provides a means of reducing the object surface variations and selectively highlighting particular defect(s) on circuit boards. The level of intensity of illumination is controlled. The invention provides a larger range of illumination angles and a higher level of irradiance than previous systems. The larger range of illumination angles minimizes the number of false defect calls resulting from spurious surface damage such as stains and minor scratches. Prior knowledge of the optical properties of an ideal of the particular circuit board, the reflectivity and direction of the defect scattering patterns, and the defect sensitivity level desired is used to predetermine the norm and to choose incidence angles at which to set the illumination intensity. Initial calibration only is required for a given workpiece. The calibration for a specific PCB configuration is stored in the computer. Because of the degree of automation in the present system, the number of instances in which direct human intervention is required is kept to a minimum. As a result, the potential for error introduced by direct human intervention and the reduction in throughput which is caused by dependency on direct human intervention in the inspection process is avoided.